Photocatalysis definition

This review has been written in order to clarify fundamental aspects of photocatalysis, an important subject in inorganic and material chemistry, not to present a list of studies on photocatalysis reported so far, since it seems rather difficult to make a complete review by introducing all or a large part of the reported studies on photocatalysis of relatively long history. This review is based on the author s experience in studies on photocatalysis and topics are limited to so-called semiconductor photocatalysis definition and examples of photocatalysis, its principle and kinetics, visible light-induced photocatalysis, and design of active photocatalysts are discussed in detail. 

However, the pathways for these reactions, particularly in the gas phase, have been only -.rtially characterized. In a wide variety of these reactions, coordinatively unsaturated, highly reactive metal carbonyls are produced [1-18]. The products of many of these photochemical reactions act as efficient catalysts. For example, Fe(C0)5 can be used to generate an efficient photocatalyst for alkene isomerization, hydrogenation, and hydrosilation reactions [19-23]. Turnover numbers as high as 3000 have been observed for Fe(C0)5 induced photocatalysis [22]. However, in many catalytically active systems, the active intermediate has not been definitively determined. Indeed, it is only recently that significant progress has been made in this area [20-23]. 

Very recently (1), in an attenpt to overcome the confusion surrounding the definition of photocatalysis, considering the contributions by Salomon 2), Moggi (3), W ighton (4), Carassiti (5),... 

Bavykin, Dmitry V. is a Ph.D. researcher in the Laboratory of photocatalysis on semiconductors at the Boreskov Institute of Catalysis, Novosibirsk, Russia. The title of his PhD thesis (1998) Luminescent and photocatalytic properties of CdS nanocolloids . Area of his interests is the photophysical-photochemical properties of nanosized sulfide semiconductors, including synthesis of particles with definite size and surface properties, their characterisation the study of the photoexcited states dynamics, relaxation in quantum dots by the luminescence and flash photolysis measurements studies of the interfacial charge transfer from colloidal semiconductor particles by the steady state photolysis, luminescence quenching method. 

Studies on heterogeneous photocatalysis have been undertaken extensively worldwide, employing significantly diverse experimental conditions including illumination, photocatalyst preparation, and reactor design. To allow the comparison of experimental data between different research laboratories, a unified, unambiguous definition of the efficiencies of photocatalytic processes is compulsory. 

Serpone N, Emeline AV (2002) Suggested Terms and Definitions in Photocatalysis and Radiocatalysis, Int. J. Photoenergy 4,... 

Since photocatalysis was discovered in the early 1970s, more than 6,200 papers related to this process have been published. Most of the work on this subject has focused on showing that organic molecules can be oxidized in PC reactors. So far, more than 800 organic molecules have been tested for oxidation in PC reactions (Blake, 2001). In most cases, the tested organic molecules were converted to CO2, water, and mineral acids. Therefore, it can be definitely concluded that photocatalysis works for oxidation of organic molecules. The rate of oxidation depends on several factors that will be addressed in the upcoming section. 

PHOTOCATALYSIS BY INORGANIC SOLID MATERIALS REVISITING ITS DEFINITION, CONCEPTS, AND EXPERIMENTAL PROCEDURES... 

The principle of photocatalysis is often explained with an illustration like Fig. 2, a schematic representation of the electronic structures of semiconducting materials, a band model. An electron in an electron-filled valence band (VB) is excited by photoirradiation to a vacant conduction band (CB), which is separated by a forbidden band, a band gap, from the VB, leaving a positive hole in the VB (Section III.B). These electrons and positive holes drive reduction and oxidation, respectively, of compounds adsorbed on the surface of a photocatalyst. Such an interpretation accounts for the photocatalytic reactions of semiconducting and insulating materials absorbing photons by the bulk of materials. In the definition of photocatalysis given above, however, no such limitation based on the electronic structure of a photocatalyst is included. For example, isolated... 

A significant problem in studies on photocatalysis is the definition of positive hole. Positive hole is defined as a defect of an electron (i.e., a positive hole must be included in a substance, while an electron is a real substance). Therefore, not only h produced by photoinduced band-to-band transition in solid materials but also a hydroxyl radical, which is a one-electron deficient hydroxyl anion, can be a positive hole. If this definition is accepted, there should be no difference in the photocatalsrtic oxidation mechanisms between direct hole transfer and surface-adsorbed hydroxyl radical reaction, since it is well known that the surface of a metal oxide is covered with chemically or physically adsorbed water and a positive hole passing through this water layer into a solution may be a hydroxyl radical or its protonated or deprotonated species (Fig. 4). Actually, hydroxyl... 

This review deals with the applications of photolurainescence techniques to the study of solid surfaces in relation to their properties in adsorption, catalysis, and photocatalysis, After a short introduction, the review presents the basic principles of photolumines-cence spectrosajpy in relation to the definitions of fluorescence and phosphorescence. Next, we discuss the practical aspects of static and dynamic photoluminescence with emphasis on the spectral parameters used to identify the photoluminescent sites. In Section IV, which is the core of the review, we discuss the identification of the surface sites and the following coordination chemistry of ions at the surface of alkaline-earth and zirconium oxides, energy and electron transfer processes, photoluminesccncc and local structure of grafted vanadium oxide, and photoluniinescence of various oxide-... 

As implied above, there is nothing dramatically special about photocatalysis. It is simply another type of catalysis alongside, as it were, redox catalysis, acid-base catalysis, enzyme catalysis, thermal catalysis and others. Consequently, it is worth reemphasising that any description of photocatalysis must correspond to the general definition of catalysis. This said, it could be argued that the broad label photocatalysis simply describes catalysis of a photochemical reaction. 

In a broad sense then, the label photocatalysis describes a photochemical process in which the photocatalyst accelerates the process, as any catalyst must do according to the definition of catalysis. 

The lack of a really precise definition of the field of organocatalysis makes it difficult to properly categorize all the catalysts and processes that can be included under this umbrella. This leads to some areas of work being included in some cases but not in others. Three such areas are photocatalysis, imprinted polymers and dendrimers. All of them have very specific characteristics and enough entities to be most often considered independently, even if, formally, many of the processes included there could be classified as organocatalytic (metal free). We will not deal in detail with those areas but, instead, include here a brief outline in the context of organocatalysis. 

No general agreement exists on the definition of the term photocatalysis. Generally it implies that light and a catalyst are necessary to induce or accelerate a chemical transformation [1-9]. This definition includes photosensitization, a process by which a photochemical alteration occurs in one molecular entity as a result of initial absorption of radiation by another molecular entity called photosensitizer [10], but it excludes the photoacceleration of a stoichiometric thermal... 

Heterogeneous photocatalysis on metal oxide semi-conductors has been shown to be effective in degrading organic pollutants in gaseous and aqueous streams (Fox and Dulay, 1993 Hoffmann, et al., 1995). In photocatalysis, the definition of energy yield parameters describing the light utilization efficiency is very critical (Fox, 1988). 

Quantum parameters are important and useful efficiency estimators in photocatalysis (Cabrera et aL, 1994). These parameters are based on a number ratio , either of photoconverted molecules over absorbed photons or photoconverted molecules over photons entering the reactor, as described in Table 6.1 Using this idea as the basis there are several possible quantum yield definitions ... 

Alternatively and as described in Chapter VI, other less meaningful yet useful definitions of PTEF in simultaneous oxidation-reduction photocatalysis leads to the following ... 

Chapter VI indicates that photocatalysis can benefit from the definition and use of efficiency parameters, which are based on fundamentals thermodynamic and kinetic properties. Such parameters permit the direct comparison of results obtained from different experimental systems and conditions. 

Chapters 1 and 2 of Part A PREFACE introduce into definitions, classifications, history, properties and biological systems of macromolecular metal complexes. Then part B SYNTHESIS AND STRUCTURES contain at first in chapter 3 kinetics and thermodynamics of formation of these complexes. The following chapters 4 till 8 describe in detail the various synthetic routes for the preparation of macromolecular metal complexes. Part C with chapters 9 till 14 is devoted to PROPERTIES. The most important ones are binding of small molecules, physical and optical sensors, catalysis, photocatalysis and electron/photon induced processes. In chapter 15 few closing remarks are made. 

For example, the quantum yield (QY or d>) is a fundamental parameter in heterogeneous photocatalysis, whose definition should be carefully considered. A standard definition has been given in terms of a particular reaction, with a defined reactant or product, at a given wavelength A. [78] ... 

From the definition of it is obvious that, both when 5- 0 (only photocatalysis) and when (only pervaporation), the intensification factor... 

This chapter introduces some properties and definitions of semiconductor materials used in heterogeneous photocatalysis. The comparison of heterogeneous photocatalytic systems and a brief description of the types of membranes that can be used is also reported. Some aspects of membrane operations such as fouling, separation of a photocatalyst and effectiveness of photodegradation on permeate quality are discussed. 

Photocatalysts are materials that induce photocatalytic reaction under photoirradiation. A general definition of photocatalysis, a conceptual name of photocatalytic reactions, is a chemical reaction induced by photoabsorption of a sohd material, or photocatalyst, which remains tmchanged during the reaction. Therefore, photocatalyst should act catalytically, i.e., without change, tmder fight. Although molecules or... 

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